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COMPARING MENDELIAN AND SEX-LINKED TRAITS OF
DROSOPHILA MELANOGASTER
By
JAMES BESS
ANGELA KHONG
DAMIAL FLETCHER
TIMOTHY PINDELL
MARCUS GAINES
JASMINE VAUGHN
JESSICA HARLIN
MICHAEL VILLENA
DAVID HUFFER
WAHLIA WALKER
September 26, 2008
Teacher: Ms. Ashli Myers
INTRODUCTION
Genetics is the study of how parents pass genes to offspring through a process known as
genetic inheritance. The two basic types of inheritance are Mendelian and sex-linked. The type
of inheritance depends on where genes are physically located. Sex-linked inheritance occurs
when genes are found on the sex chromosomes. Mendelian inheritance occurs when genes are
found on the autosomes (Klug, Cummings, and Spencer, 2006).
A gene is a section of DNA that controls a trait in an organism. Genes are located on
DNA, which is compacted to form chromosomes (Fig. 1). Organisms inherit one set of
chromosomes from each parent; therefore, they inherit one set of genes from each parent
(Joseph, Miller, and Kenneth, 1998). The inherited genes have multiple forms, which are called
alleles. For example, the gene for height can have one of two alleles, a tall allele (T) which is
dominant, or a short allele (t) which is recessive. Traits expressed by the offspring depend on the
alleles inherited from the parents (Joseph, Miller, and Kenneth, 1998).
Fig. 1 – Figure 1 shows a picture of a chromosome, DNA, and a gene.
An organism receives one allele from its mother and one allele from its father. The
dominant allele masks or hides the recessive allele. As can be seen in Fig 2, individual A will
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express the dominant trait because it is heterozygous; it has one dominant (T) and one recessive
(t) allele. Individual B will express the dominant trait because it is a homozygous dominant; it
has two dominant alleles (TT). Individual C is a homozygous recessive organism because it has
two recessive alleles (tt). Individual C will express the recessive trait (Schraer and Stoltze, 1999).
Fig. 2 – Figure 2 shows the difference between heterozygous and homozygous
Genes are found on chromosomes; the two types of chromosomes are sex chromosomes
and autosomes (Solomon, Berg and Martin; 2002). All organisms have one set of sex
chromosomes. The number of autosomes varies per organism. For example, fruit flies and
humans have one sex chromosome, but fruit flies have three autosomes while humans have
twenty-two (Fig.3). Sex chromosomes determine the gender of the organism; autosomes do not
determine gender but do determine traits. The genes that are located on autosomes are transferred
through Mendelian inheritance. Genes that are located on sex chromosomes are transferred
through sex-linked inheritance (Mittwoch, 1967).
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A
B
Fig. 3 – Section (A) shows the Human karotype and Section (B) shows the fruit flies karotype
Most chromosomes are autosomes. Genes located on the autosomes are passed through
Mendelian inheritance. Through Mendelian inheritance, each offspring receives two alleles, one
from the mother and one from the father. The Punnett square is used to determine the possible
allele combinations resulting from a parent cross. A Punnett square is also used to determine the
phenotypic ratio of the offspring. A phenotypic ratio is the ratio of visible traits displayed in the
offspring. The Punnett square in Figure 4 shows the different allele combination and the
phenotypic ratio that will result when two heterozygous parents are crossed. The offspring will
either be homozygous dominant (TT) expressing the dominant trait, or heterozygous (Tt)
expressing the dominant trait, or homozygous recessive (tt), expressing the recessive trait. The
offspring has a 3:1 phenotypic ratio; three dominant and one recessive.
T
t
T
TT
Tt
t
Tt
tt
Fig. 4- This figure shows the Mendelian parent cross and the offspring
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In sex-linked inheritance, however, every offspring does not necessarily get two alleles.
There are two types of sex-chromosomes, the X chromosome and Y chromosome. A female has
a pair of X chromosomes, one from the mother and one from the father. The male has an X and a
Y chromosome. The Punnett square in Figure 5 shows the female with a pair of X chromosomes
and a male with an X and Y chromosome. The X chromosome is from the mother and the Y
chromosome is from the father. The Y chromosome is very small and does not hold many genes,
whereas the X chromosome holds most of the genes. When the gene is found on the X
chromosome and not the Y chromosome, then the female offspring receives two X chromosomes
which means the female inherits two alleles. The male offspring only receives one allele because
the male only has one X chromosome. For example, when a heterozygous female is crossed with
a dominant male, the offspring will be females with two alleles and males with one allele. The
females will be homozygous dominant (XT XT), and heterozygous (XT Xt), while the males will
be dominant (XTy), and recessive (Xty) (Fig. 6).
X
X
X
XX
XX
y
Xy
Xy
Fig. 5- This figure shows the possible gender outcomes of a cross between a male and female parent
XT
Xt
XT
XT XT
XT Xt
y
XTy
Xty
females with two alleles
males with only one allele
Fig. 6- This figure shows the sex-linked parent cross and the offspring
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The purpose of the study was to use the ratios in the F2 generation to determine whether
the genes in Drosophila melanogaster for eye color and wings are found on the autosomes,
resulting in Mendelian inheritance, or the sex chromosomes, resulting in sex-linked inheritance.
When a trait is passed through Mendelian inheritance, a cross between a homozygous dominant
male (TT) and homozygous recessive female (tt) will result in all offspring being heterozygous
in the F1 generation (Fig. 7). The F2 generation is produced by crossing the heterozygous
females from theF1 generation with the heterozygous males from the F1 generation. The F2
generation will have a phenotypic ratio of 3(dominant): 1(recessive) (Fig. 8).
T
T
T
Tt
Tt
T
Tt
Tt
Fig. 7- This figure shows the
Mendelain parent cross and the
resulting F1 generation.
T
t
T
TT
Tt
t
Tt
tt
Fig. 8- This figure shows the
F1cross and the resulting F2
generation
When a trait is passed through sex-linked inheritance and the gene for the trait is found
only on the X chromosome, a cross between a dominant male (XTy) and a homozygous recessive
female (XtXt) will result the F1 generation. The females in the F1 generation will have two
alleles and will be heterozygous. The male offspring in the F1 generation will be recessive with
only one allele (Fig. 9). The F2 generation is produced when the heterozygous females from the
F1 generation are crossed with the recessive males from the F1 generation. The F2 generation
will have a phenotypic ratio of 1:1:1:1(one heterozygous female (XTXt) to one homozygous
recessive female (XtXt) to one dominant male (XTy) to one recessive male (Xty) (Fig. 10).
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Xt
Xt
XT
XTXt
XTXt
y
Xty
Xty
XT
y
XT
XTXt
XTy
Xt
XtXt
Xty
Fig. 10 – This figure shows the F1
cross and resulting F2 generation.
Fig. 9- This figure shows the
parent cross and resulting F1
generation
The difference between sex-linked and Mendelian inheritance is the phenotypic ratio in
the F2 generation. Because most genes are found on autosomes, most traits are passed through
Mendelian inheritance. The null hypothesis states that the genes for eye color and wings in
Drosophila melanogaster are passed through Mendelian inheritance and will result in a 3:1
phenotypic ratio in the F2 generation. The alternative hypothesis states that the genes for eyes
and wings in Drosophila melanogaster are passed through sex-linked inheritance rather than
Mendelian inheritance, and will result in a 1:1:1:1 phenotypic ratio in the F2 generation.
METHODS
Drosophila melanogaster or fruit flies are good organisms to use in the research of
genetics. Fruit flies are used in genetics and biological research because fruit flies are
inexpensive and easily cultured. Fruit flies also have high fecundity rates; a female fruit fly can
lay up to 500 eggs in 10 days.
In order to test the hypothesis, the students had to observe the F2 generation. Because the
fruit flies take two weeks to hatch and the group only had six weeks to finish the experiment, the
instructor did the initial parent cross two weeks prior to the students arriving. The instructor
crossed a homozygous recessive female (tt) with either a homozygous dominant male (TT) or
dominant male with only one allele (XTY). This cross produced the F1 generation which crossed
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and produced the F2 generation. The students analyzed the F2 generation to determine what kind
of male was crossed with the females.
On June 26th the culture vessels were prepared by following the instructions in the
Carolina Drosophila Manual. One part Drosophila medium was deposited into the bottom of the
culture vessels with one part water. Then the medium was left to firm. Once the medium was
firm, the yeast was added. Medium and yeast are used as a food source; medium is for the larva
to eat, and yeast is for adult flies to eat. A piece of plastic mesh was added to the culture vessel
to give the fruit flies more surface area, which was needed so that they would not crush one
another or get stuck in the medium.
To sex the fruit flies, the flies were first transferred to an empty culture vessel to be
anesthetized with FlyNap®. Then the flies were moved to a wide field stereomicroscope and
separated by gender. Male fruit flies have heavy dark bristles around the base of the genitalia,
and females do not.
After the F1 flies were sexed, six males and six females were put into a culture vessel to
breed the F2 generation. Since there was not enough time to wait for F2 generation to hatch
naturally, the culture vessels were put in an incubator at 28.5 °C to speed up the process of
development. After the F1 flies reproduced, they were released so they would not be counted
with the F2 generation. As the F2 flies hatched over a seven day period they were counted and
categorized. The flies were then categorized by traits and gender. The wings groups categorized
by male with wings, males without wings, females with wings, and females without wings. The
eyes group categorized by males with red eyes, males with white eyes, females with red eyes,
and females with white eyes.
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A table was created to show the totals of each category: wings, no wings, red eyes and
white eyes. The group performed a Chi square test to compare the observed results to the
expected Mendelian ratio of 3:1 and the expected sex-linked ratio of 1:1:1:1 in the F2 generation.
A Chi square test tells whether there is a significant difference between the expected and
observed results or if the difference is due to chance.
RESULTS
Table 1 shows the number of observed flies compared to expected flies with and without
wings. The expected ratio was 3 (flies with wing): 1 (fly without wings) as predicted by the
Mendelian inheritance cross. The observed ratio was 5 (flies with wings): 1 (fly without wings).
The Chi square test compared the ratios, the result of which was a p-value of much less than
0.01. This meant that there was very close to 0% probability that the difference was due to
chance, and that there was a significant difference between the observed and expected ratios.
Based on the data, the gene for wings is not passed through Mendelian inheritance (Table 1).
Table 1. – This table shows the winged and non winged observed 5:1 and expected 3:1 ratios for the Mendelian
inheritance.
Expected
3:1
Wings
Observed
5:1
343
No wings
69
103
Total
412
412
309
Table 2 shows the comparison of observed to expected female to male flies with and
without wings. The expected ratio was 1 (male with wings): 1 (male without wings): 1 (female
with wings): 1 (female without wings), as predicted by the sex-linked inheritance cross. The
expected ratio was also based on a 1:1 ratio of females to males. The observed ratio was 11
(males with wings): 3 (males without wings): 9 (females with wings): 1 (female without wings).
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The Chi square test resulted in a p-value of much less than 0.01, showing that there was a
significant difference between the observed and expected ratios. Based on the data, the gene for
wings is not passed through sex-linked inheritance (Table 2).
Table 2. – This table shows the winged and non winged observed 11:3:9:1and expected 1:1:1:1 ratios for the sexlinked inheritance.
Males Wings
Observed
11:3:9:1
189
Expected
1:1:1:1
103
Males No Wings
51
103
Females Wings
Females No
Wings
154
103
18
103
Table 3 compares the observed and expected male to female ratios. The expected ratio
was 1 (female): 1 (male) as predicted any time a male and female reproduce. The observed ratio,
however, was not 1 (female): 1 (male). The Chi square test resulted in a p-value of much less
than 0.01, showing that there was a significant difference between the observed and expected
ratios (Table 3). Because of the results of the Chi square test, the decision was made to use the
observed ratio when comparing for sex-linked inheritance.
Table 3. - This table shows the observed and expected female to male ratios.
Observed
Expected
Females
172
206
Males
240
206
Total
412
412
Table 4 shows the comparison of observed to expected female to male flies with and
without wings. The expected ratio was modified to reflect the female to male ratio actually
obtained. The observed ratio was 11 (males with wings): 3 (males without wings): 9 (females
with wings): 1 (female without wings). The Chi square test resulted in a p-value of much less
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than 0.01, showing that there was a significant difference between the observed and expected
ratios. Based on the data, the gene for wings is not passed through sex-linked inheritance.
Table 4. – This table shows the winged and non winged observed 11:3:9:1 and expected 1:1:1:1 for the second sexlinked.
Males Wings
Males No Wings
Females Wings
Females No
Wings
Total
Observed
189
51
154
Expected
119
119
87
18
87
412
412
Table 5 shows the number of observed flies compared to the expected flies with red or
white eyes. The expected ratio was 3 (flies with red eyes): 1 (fly with white eyes) as predicted by
the Mendelian inheritance cross. The observed ratio was 1 (fly with red eyes): 1 (fly with white
eyes). The Chi square test resulted in a p-value of much less than 0.01, showing that there was a
significant difference between the observed and expected ratios. Based on the data, the gene for
eye color is not passed through Mendelian inheritance (Table 5).
Table 5. – This table shows the red eyed and white eyed observed 1:1 and expected 3:1 ratios for the Mendelian
inheritance.
Red Eyes
White Eyes
Total Flies
Observed
1:1
426
421
847
Expected
3:1
635
212
847
Table 6 shows the comparison of observed to expected female to male flies with red or
white eyes. The expected ratio was 1 (male with red eyes): 1 (female with red eyes): 1 (male
with white eyes): 1 (female with white eyes), as predicted by the sex-linked inheritance cross.
The expected ratio was also based on a 1:1 ratio of females to males. The observed ratio was 1
(male with red eyes): 2 (females with red eyes): 1 (male with white eyes): 1 (female with white
eyes). The Chi square test resulted in a p-value of much less than 0.01, showing that there was a
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significant difference between the observed and expected ratios. Based on the data, the gene for
eye color is not passed through sex-linked inheritance.
Table 6. – This table shows the red eyed and white eyed observed 1:2:1:1 and expected 1:1:1:1 ratios for the sexlinked inheritance.
Red Eyes Male
Red Eyes Female
White Eyes Males
White eye Females
Observed
1:2:1:1
167
259
177
244
Expected
1:1:1:1
212
212
212
212
Table 7 compares the observed and expected male to female ratios. The expected ratio
was 1 (female): 1 (male), as predicted by any time a male and female reproduce. The observed
ratio was 2 (females): 1 (male). The Chi square test resulted in a p-value of much less than 0.01,
showing that there was a significant difference between the observed and expected ratios (Table
3). Because of the results of the Chi square test, the decision was made to use the observed ratio
when comparing for sex-linked inheritance.
Table 7. - This table shows the observed 2:1 and expected 1:1 female to male ratios.
Females
Observed
2:1
503
Expected
1:1
424
Males
344
424
Table 8 shows the comparison of observed to expected female to male flies with red or
white eyes. The expected ratio was modified to reflect the female to male ratio actually observed.
The observed ratio was 1 (male with red eyes): 2 (females with red eyes): 1 (male with white
eyes): 1 (female with white eyes). The Chi square test resulted in a p-value of 0.86422836,
showing that there was an 86% probability that the difference was due to chance, and that there
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was not a significant difference between the observed and expected ratios. Based on this data, the
gene for eye color is passed through sex-linked inheritance.
Table 8. – This table shows the red eyed and white eyed observed 1:2:1:1 and expected 1:1:1:1 for the second sexlinked.
Red Eyes Male
Red Eyes Female
White Eyes Males
White eye Females
Total Flies
Observed
1:2:1:1
167
259
177
244
847
Expected
1:1:1:1
172
252
172
252
847
DISCUSSION AND CONCLUSION
The purpose of the study was to use the ratios in the F2 generation to determine whether
the genes in Drosophila melanogaster for eye color and wings are found on the autosomes,
resulting in Mendelian inheritance, or the sex chromosomes, resulting in sex-linked inheritance.
The null hypothesis stated that the genes for eye color and wings were Mendelian inherited and
would result in a phenotypic 3:1 ratio in the F2 generation. The alternative hypothesis stated that
the gene for eye color and wings was not Mendelian but sex-linked, and would result in a
phenotypic 1:1:1:1 ratio in the F2 generation.
The wings group rejected both the null hypothesis and alternative hypothesis. Based on
the p-values that were acquired from the Chi-squared tests, the data showed that the observed
ratios were all significantly different from the expected Mendelian ratio and the expected sexlinked ratio.
The eyes group rejected the null hypothesis because there was not a 3:1 ratio based on the
p-value, which was much less than 0.01 in the Chi square test for Mendelian inheritance. The
alternative hypothesis was accepted because of the sex-linked ratio was found when using the
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observed male to female ratio. The resulting p-value was much greater than 0.01 in the Chi
square test for sex-linked inheritance, indicating that the gene for eye color was sex-linked.
The wing group did not obtain a 3:1 Mendelian ratio or a 1:1:1:1 sex-linked ratio because
the wing group obtained significantly (five times) more winged flies than non-winged flies.
When the flies with no wings hatched some of the flies got stuck in the medium because they
could not fly. The group did not count the flies stuck in the medium and this led to incomplete
results and a small sample size. The other reason why the wing group obtained a small sample
size is that the students were inexperienced at handling and transferring the flies. The wing group
collected 412 flies whereas the eye color group collected 847 flies; the wing group collected less
than half of what the eye color group collected.
The students in the wing group and the eye color group did not obtain a 1(male): 1
(female) ratio. The wing group counted more male than female flies, and the eye color group
counted more female than male flies. A reason that the wing group counted more male than
female flies, and specifically more non-winged males than non-winged females, is that since the
females emerged before the male flies, more non-winged females probably got stuck in the
medium than the non-winged males. The non-winged flies were more likely than the winged
flies to get stuck in the medium they could not fly into the empty culture vessel used to collect
live flies for data collection during transfer.
The reason that the eye color group counted more female than male flies could be that the
females hatch first. If more time was allotted, then all of the flies would have been able to hatch,
and the group would have been able to count all of the flies. In all likelihood, most of the male
flies were still not hatched. Other reasons that the two groups did not obtain a 1 (male): 1
(female) ratio could be that inexperienced students incorrectly identified the male and the female
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flies, or that the flies escaped while handling them. The students only had one day to learn how
to handle the flies.
The main limitation to the study was time. Not all the F2 generation flies hatched during
the experiment. There was still some pupa incased in the culture vessel when the group stopped
collecting the data. This could have altered the male to female ratio because the females hatch
first; the remaining flies were mostly likely males. Incubation became a factor because the group
did not have enough time. If the group had more time, then the flies could have hatched
naturally, which is better than incubation. When the group incubated the flies, the lifecycle sped
up. This may have caused an unhealthy development, and premature deaths and dead flies were
stuck in the medium. The flies stuck in the medium were difficult to remove, so they were not
counted. The group was inexperienced because there was only one day to practice handling the
flies. This was also the students first time transferring flies, making culture vessels and sexing
the fruit flies. Inexperienced students may have allowed a fly to escape from the culture vessel
when transferring the flies. If the group practiced transferring the flies more, the flies may not
have escaped and the group would have a bigger sample size. In addition, a male fly could have
been sexed incorrectly if the fly had a lighter spot than usual on its genitalia. Some of the culture
vessels might not have been prepared correctly, and the medium could have been too dry.
Therefore in some culture vessels flies died and the sample size reduced because the females
could not lay their eggs in dry medium to make more flies; and some culture vessels were
useless.
An improvement to the study could be that the group could have extracted all the flies.
This would include the dead flies stuck in the medium and the flies that escaped, both of which
were not counted. The dead flies stuck in the medium were not counted, but could have been
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counted by extracting the medium after all the flies were released. Another improvement could
allow them to hatch naturally instead of through incubation. Making more usable culture vessels
also could have yielded a bigger sample size.
Some questions arose when the group was performing the study. Did the incubator help a
specific gender hatch faster? Does incubation lower or raise the fecundity rate? How would
different culture vessels with different amounts of medium effect the growth of the fruit flies?
Did the specific species of fruit fly make a difference to the outcome of the experiment?
The study of genetics is important to helping people understand how traits are passed
down through generations, and to predict genetic diseases and disabilities. This study of
Mendelian and sex-linked inheritance is only a very small part of what is actually being done in
this increasingly important field. The more people can learn about genetics, the more knowledge
they can gain about the prevention and elimination of negatively inherited traits.
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BIBLIOGRAPHY
Flagg, R. O., Ph. D (1988). Carolina drosophila manual. Burlington, North Carolina: Carolina
Biological Supply Company.
Joseph, S, Miller, L, & Kenneth, R (1998). Prentice Hall: Biology. Lebanon: Prentice Hall.
Klug, W., Cummings, M., & Spencer, C., 2006. Concepts of genetics eight edition. Upper
Saddle: Pearson Education
Mittwoch, U. (1967). Sex Chromosomes. New York, London: Academic Press
Schraer, W. D., & Stoltze, H. J. (Eds.). (1999). Biology, The study of life. Upper Saddle River,
New Jersey: Prentice Hall.
Solomon, E. P., Berg, L. R., & Martin, D. W. (2002). Biology. Thomas Learning Inc..
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